Hi,
You'll need to use one of the test versions of VMD 1.9.2 since you have
upgraded your GPU hardware to a Kepler-based GPU chip (GeForce GTX 660 Ti).
The VMD 1.9.1 release pre-dates Kepler and didn't have the right compilation
flags to support Kepler, so to cure this problem, just grab one of the
recent test builds of VMD 1.9.2 here:
http://www.ks.uiuc.edu/Research/vmd/alpha/

We will soon begin releasing the first beta releases of VMD 1.9.2,
and a few weeks after that will be the official release. We run the
test versions regularly here in our lab, so there's no reason to fear
running one of these builds of VMD to address your problem in the
short-term.